It is known that differential amplifiers should receive first and second signals at their respective first and second inputs and to generate an output which is solely a function of the difference between the first and second signals. If the signals have a common mode component, then the value of this common mode component should not affect the output from an ideal differential amplifier.
However, in real devices component mismatch means that the devices deviate from the ideal response characteristic and that the output shows some dependence on the common mode input voltage. There is therefore a need to improve the performance of circuits using differential amplifiers. The amount by which a common mode input signal manifests itself as a change in the output signal is often referred to as the common mode rejection ratio. It is, in essence, a measure of how well the amplifier avoids mixing the common mode signal with the differential signal.